Ronnie O'Sullivan took a 10-7 lead over Ali Carter on day one of the World Championship final at the Crucible.

Three-time champion O'Sullivan made two century breaks, including a magnificent 141, to lead fellow Essex player Carter 5-3 after the first session.

Carter's resolve appeared to be creaking after the restart, with O'Sullivan capitalising on a string of mistakes to stretch his lead to four.

But Carter kept grinding away, winning a crucial final frame to stay in touch.

"Ali Carter has shown a lot of good qualities to stick in there. He's closed the gap and is still in it. Ronnie O'Sullivan has shown flashes of brilliance but not his top form, while Carter is getting chances but not scoring enough.

"Tactically he's pretty sound but you have to make the most of your chances and put your opponent under pressure by winning a frame at one visit. He's not firing on all cylinders. I don't think Ronnie will be totally happy. He will think he should have a bigger lead."

This year's final is a repeat of the 2008 decider, which O'Sullivan won 18-8 to secure his third world title, while Carter has never beaten O'Sullivan in 11 previous attempts in ranking events.

While Carter's displays before the final suggested he would be a tougher nut to crack than four years ago, O'Sullivan, who has demonstrated unerring focus over the past two weeks, went into the rematch a red hot favourite.

O'Sullivan, 36, has strung together six straight frames against every player in every round so far this year, while his pre-final threat to quit the sport, win or lose, has added extra significance to the occasion.

The omens did not look positive for Carter in the first two frames, with O'Sullivan carrying on from where he left off in his 17-10 semi-final victory over Matthew Stevens.

After a tense tactical exchange, Carter, who overcame Stephen Maguire in his semi-final on Saturday, finally broke the tension when he potted the first ball of the match with 11 minutes and 44 seconds on the clock.

"Ali Carter's kept himself in contention. He's lost the first two sessions and he looked on the brink of losing. He was completely outplayed but Ronnie O'Sullivan has not put him to bed. If the game changes - if Carter can raise his up and O'Sullivan drops down - then Carter is not too far behind. Carter has given himself a chance should the equation of the match change."

However, a foul let his rival in and a break of 56 proved enough to take the opener, before O'Sullivan made a run of 117, his 10th century of the tournament, to make it 2-0.

But Carter rallied, compiling a nuggety break of 84 to make it 2-1 before winning a sticky, 35-minute frame to draw level at the mid-session interval.

After the break, O'Sullivan, his naturally aggressive game being tempered by his rival's obdurate style of play, nicked another tense frame to retake the lead before Carter took advantage of a couple of slip-ups to level again.

A remarkable 92 clearance from O'Sullivan - described by commentator Dennis Taylor as "one of the finest breaks you will ever wish to see" - made it 4-3 before he broke the shackles completely with a magnificent clearance of 141, his highest break of the tournament so far, to lead by two frames after the first session.

Carter, 32, reduced the deficit immediately after the restart, only for O'Sullivan to reel off the next two frames courtesy of runs of 49 and 68.

"It's been tough, hard matchplay snooker. Both players will be reasonably happy with the score. It could have been a lot worse for Ali Carter while Ronnie will be pleased to be three frames clear."

Carter, who has made much of the mental coaching he has been receiving from 2002 champion Peter Ebdon, refused to buckle, drawing on all his reserves to win a pivotal 12th frame after O'Sullivan potted the white.

However, there were signs Carter was finally losing his grip in the next two frames. Having missed two regulation blacks to hand his opponent an 8-5 lead, he made a series of mistakes in the next frame and opted not to return to the table with only one snooker needed as O'Sullivan moved four clear.

Carter steadied the ship with a battling break of 59 to make it 9-6 before O'Sullivan made a fluent run of 62 to reinstate his four-frame lead.

And while Carter took a scrappy final frame and has partially succeeded in shackling his opponent so far, few people will be betting against O'Sullivan returning on Monday to seal his fourth world crown.